1. Field of the Invention
The invention relates to a microstructure reactor for carrying out catalytic reactions.
2. Discussion of Background Information
Microstructure reactors have already been implemented in various embodiments and are already being used commercially in a micro-process technique for certain applications. They are designed with special consideration of microtechnological boundary conditions. A microstructure reactor comprises at least, but not exclusively, a reaction zone with at least one inflow and at least one outflow. Controlled reactions take place in the reaction zones, whereby a catalyst is used in at least one reaction zone. An embodiment is also possible without catalyst. As a basic principle the reaction zones can be designed as mixing wells or as continuous-flow wells with merging of fluids and/or branching.
Usually a distinction is made between simple cross-flow and counter-flow or cocurrent-flow processes. In many cases there is a cross-flow-like share. If coolants are used that do not pass through a phase change, the result is an uneven capacity for the cooling of the different reaction channels within a plate of the overall system. The number of reaction channels passed over is different for each cooling channel. The rise in temperature leads to a higher temperature emerging in the cooling channel underneath. Owing to the exponential increase in the reaction rate of the chemical reaction with the temperature in the particular reaction channel this leads to an additional discrepancy between the cooling capacity of the coolant and the ability of the reaction to evolve heat. Moreover, the viscosity of the fluids changes, which again leads to an uneven distribution of the media over the individual channels and on both the cooling plate and the reaction plate, which is also undesirable. In this connection the uneven distribution of the reaction medium is a challenge, because this automatically means a different residence time.
In the case of evaporative cooling the facts are even more complicated. Just as when coolants are used that remain in a single phase, the heat to be removed changes along the reaction channel. In respect of an at least partially present cross-flow arrangement this means a different degree of evaporation. This is undesirable with regard to the use of the generated steam for other subsystems in an overall process for increasing the efficiency of the overall process chain. This even distribution is additionally influenced negatively with regard to the achievable overall degree of steam of the coolant. Thus, steam is evolved first over areas in which the reaction progresses more quickly. The increase in the speed of the coolant that occurs there leads to a reduction in the throughput of the specific cooling channel through communication regarding the pressure in the overall system and thus reinforces the effect of the different degree of evaporation of the coolant between the channels of a plate. In addition temperature control of the reaction is made more difficult. The reaction channels over which pure steam passes can no longer be cooled adequately, because the mass flow and the specific heat capacity of the steam are considerably smaller.
Furthermore, the generally usual area and volume requirement of the catalytic reaction (>90% of all cases) is considerably greater than the channel surface that is required, in order to carry away the heat of reaction. That means the plane with cooling channels is usually severely oversized. In other words, the possible heat-transfer coefficient is higher than required, depending on one's point of view. Moreover, the calculable heat flow per pair of plates consisting of reaction and cooling is greater than the enthalpy of reaction to be transferred. This fact can additionally reinforce the effect of the locally uneven evaporation transversely to the row of the reaction channels. This is because the evaporation of the coolant can occur earlier and be completed ahead of time. In extreme cases the evaporation procedure can take place before the actual cooling channels and the distribution of the coolant in the longitudinal direction of the reaction channels can be made more difficult. Because the reaction channels are frequently oriented vertically, this means implicitly that the distribution to the cooling channels likewise takes place vertically and is influenced by gravitation. Thus, finally access to certain regions can be prevented by bubble formation in front of the cooling channels.
There are a few solutions in the prior art for solving the problems highlighted. In WO 002004017008 flow control with phase change in microchannels is described. WO 002004037418 describes the cross-flow type of construction with filling of catalysts, whereby the catalyst is graded, in order to control the heat produced. The possibility of distribution in channel structures through the influencing of pressure is known from WO 002005044442. In WO 002005075606 the process of Fischer-Tropsch synthesis with co-catalysts in microreactors (>25% cobalt loading) is presented. The document relates moreover to the possibility of using different numbers of channels along the reaction zone to cool the reaction. The possibility of temperature gradation with different coolant is known in turn from WO 002005082519. The content of WO 002005065387 is the possibility, in principle, of using a reaction zone for evaporation. The necessary measures to prevent too much deformation in respect of the slot-shaped design of microchannels through reinforcement of the side walls emerge from WO 002011075746. The distribution of reactant gas in the coolant with partial cross-flow arrangement, partial addition of reactants and heat exchange very generally are presented from WO 002012054455 and WO 2011134630. Finally, U.S. Pat. No. 6,994,829 describes the use of (tortuous) small channels for evaporation paired with subsequent superheating in straight, larger channels. The coupling of two reactions is known from U.S. Pat. No. 7,014,835 and DE 10044526. The use of the column structure for multi-phase reactions with the feed of reactants emerges from DE 102005022958. A presentation of the sequential carrying-out of catalytic reactions with intermediate cooling exists in DE 10201210344.
In none of these documents is there a description of the necessary measure in cross-flow-type arrangements for effective distribution of coolant that is to be evaporated completely, but not necessarily superheated. The partial addition is used for reactions and graduated catalyst beds/cooling zones are proposed for better cooling.
Accordingly, the object of the present invention is to remedy the problems described. More particularly the challenge of achieving an even temperature throughout the reactor as a whole by means of parallelised distribution structures remains in place.